Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel.
Diagnostic routines may be intermittently performed to verify functionality of emission control system components, such as various valves coupled to the canister. One example approach is shown by Machida et al. in U.S. Pat. No. 5,592,923. Therein, an engine intake manifold vacuum is applied on the emission control system. A reference pressure is determined based on a combination of open and close conditions of emission control system valves. Based on a difference between an estimated system pressure relative to the reference pressure, degradation of a canister purge valve (coupled between the canister and the intake manifold) may be determined. Another example approach is shown by Otsuka et al. in U.S. Pat. No. 5,295,472. Therein, an engine control system identifies degradation of a canister vent valve (coupled between the canister and the atmosphere) and degradation of the canister purge valve based on a rate of change in fuel tank pressure following application of intake manifold vacuum on the fuel tank.
However, the inventors herein have identified potential issues with such an approach. As one example, the approach of Otsuka and Machida may not accurately distinguish elevated fuel tank vacuum levels caused by a stuck closed canister vent valve from elevated vacuum caused by a leaky open canister purge valve. In addition, since the diagnostic routine is performed while the engine is running, engine vacuum noise may corrupt degradation detection results. As such, if the canister vent valve or purge valve degradation is not accurately identified, fuel tank vacuum levels may become excessive, potentially harming the fuel tank. Further, if canister vent valve and purge valve degradation are not accurately distinguished, appropriate mitigating steps may not be possible. As such, this may lead to an increase in MIL warranty.
In one example, some of the above issues may be addressed by a method for a vehicle fuel system, comprising: sealing a fuel system (from atmosphere and an engine intake) after an engine pull-down; and distinguishing degradation of a canister vent valve from degradation of a canister purge valve based on a change in fuel system vacuum following the sealing.
As an example, during engine running conditions, a fuel tank (negative) pressure may be monitored. In response to excessive fuel tank vacuum levels (e.g., fuel tank vacuum being higher than a threshold level), degradation of one of the fuel system canister purge valve and the fuel system canister vent valve may be determined. To distinguish between the two and enable appropriate mitigating steps to be taken, the fuel tank may be isolated following a subsequent engine pull-down. As such, the engine pull-down may include a vehicle key-off condition (wherein the vehicle operator has explicitly indicated a desired to shut down the engine) or may include shift of vehicle operation (in a hybrid vehicle) from an engine mode to an electric mode. Further still, an engine pull-down may occur during an idle-stop in vehicles where the engine can be selectively deactivated during idle-stop conditions. As such, following an engine pull-down, engine vacuum noise may be reduced, and fuel system valve degradation may be identified more accurately.
In particular, after the engine pull-down, a vehicle controller may isolate the fuel tank by closing the canister vent valve (to isolate the fuel tank from the atmosphere) while also closing the canister purge valve (to isolate the fuel tank from the engine intake), or while maintaining the canister purge valve closed. If the fuel tank vacuum level falls (e.g., below the threshold level) following the sealing of the fuel tank, it may be determined that the previously experienced excessive fuel tank vacuum was due to the canister purge valve being stuck open. However, if the fuel tank vacuum level remains elevated, the controller may try to actuate the vent valve open while maintaining the purge valve closed. If there is still no change in fuel tank vacuum following the actuation of the vent valve, it may be determined that the canister vent valve (e.g., the canister vent solenoid) is stuck closed. If the fuel tank vacuum gradually bleeds up (to atmospheric conditions) following the actuation of the vent valve, it may be determined that the fuel system valves are not degraded and that the elevated fuel tank vacuum may be due to a blockage in a fresh air line (that is, the canister vent).
In this way, by correlating changes in vacuum level of an isolated fuel tank with the commanded position of various fuel system valves, canister vent valve degradation and canister purge valve degradation can be identified and differentiated. By performing the diagnostics during conditions when the engine is not running, errors in degradation detection incurred due to engine vacuum noise contributions can be reduced. By improving the accuracy of degradation detection and differentiation, appropriate mitigating steps can be taken to reduce the unintended elevation of fuel tank vacuum levels. Overall, fuel system integrity can be better maintained.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.